Sodium hydroxide (NaOH) solutions are used in petroleum refining to remove hydrogen sulfide (H.sub.2 S) from various hydrocarbon streams. Once H.sub.2 S reacts with the majority of NaOH in the solution, the solution becomes known as spent-sulfidic caustic. Spent caustics typically have a pH greater than 12, sulfide concentrations exceeding 2-3 wt %, and a large amount of residual alkalinity. Depending on the source, spent caustic may also contain phenols, mercaptans, amines, and other organic compounds which are soluble or emulsified in the caustic. For example, a 20 wt % sodium hydroxide solution can be used to remove residual amounts of H.sub.2 S from natural gas after the majority of H.sub.2 S has been removed by the refinery gas plant amine unit. In this application, percent levels of amines can be found in the spent caustics due to amine carryover.
Currently, most spent-sulfidic caustics generated by refineries are either sent off-site to commercial operations for recovery or reuse (pulp and paper mills, for example) or for disposal by deep-well injection. Biological treatment in the refinery wastewater treatment unit is an inexpensive disposal option. However, many refineries do not have the wastewater treatment capacity to treat the entire amount of spent caustic generated and concerns regarding odors and toxicity frequently prohibit this practice.
Future regulatory changes are expected to result in more stringent controls and increased cost for off-site management of spent caustic. In such an event, low cost on-site treatment options would be desired. Even without regulatory changes, current off-site transportation and disposal cost warrant further investigation of on-site management alternatives. Wet-air oxidation is a commercially available process for on-site management of spent caustic, but wet-air oxidation can result in significant capital investment and high operating cost. Wet-air oxidation can be particularly expensive for spent-caustic streams from small to medium size refineries due to an insufficient economy of scale.
It has been shown that the bacterium Thiobacillus denitrificans will oxidize sulfides to sulfate. T. denitrificans is a strict autotroph and facultative anaerobe first described in detail by Baalsrod and Baalsrod ("Studies on Thiobacillus denitrificans," Arch. Mikro., 20, 34-62 (1954)). Sulfide, elemental sulfur and thiosulfate may be used as energy sources with oxidation to sulfate. Under anoxic conditions, nitrate may be used as a terminal electron acceptor with reduction to elemental nitrogen.
It has demonstrated that T. denitrificans may be readily cultured under aerobic or anoxic conditions with H.sub.2 S(g) as an energy source at pH 7.0.degree. and 30.degree. C. When H.sub.2 S (1% H.sub.2 S, 5% CO,. and balance N.sub.2) was bubbled into cultures previously grown on thiosulfate, H.sub.2 S was metabolized with no apparent lag. At loadings of 4-5 retools H.sub.2 S/h - g biomass (mmols H.sub.2 S per hour per gram biomass), H.sub.2 S concentrations in the outlet gas could be reduced to undetectable levels with 1-2 seconds of gas-liquid contact time. Under sulfide-limiting conditions, concentrations of total sulfide in the culture media were less than 1 .mu.M. Complete oxidation of H.sub.2 S to sulfate was observed (Sublette, K. L. and N. D. Sylvester, "Oxidation of Hydrogen Sulfide by Thiobacillus denitrificans. Desulfurization of Natural Gas," Biotech. Bioeng., 29(6), 249-257 (1987); Sublette, K. L. and N. D. Sylvester, "Oxidation of Hydrogen Sulfide by Continuous Cultures of Thiobacillus denitrificans," Biotech. Bioeng., 27, 753-758 (1987); Sublette, K. L. and N. D. Sylvester, "Oxidation of Hydrogen Sulfide by Mixed Cultures of Thiobacillus denitrificans and Heterotrophs," Biotech. Bioeng., 29(6), 759-761 (1987); and Sublette, K. L., "Aerobic Oxidation of Hydrogen Sulfide by Thiobacillus denitrificans," Biotech. Bioeng., 29, 690-695 (1987)).
The effect of H.sub.2 S loading on reactor performance has also been investigated. In certain experiments, the H.sub.2 S feed rate was increased in steps until H.sub.2 S breakthrough was obtained. At this point, the H.sub.2 S feed rate exceeded the rate at which the H.sub.2 S could be oxidized by the biomass. This upset condition was characterized by the accumulation of elemental sulfur and inhibitory levels of sulfide in the reactor medium. This upset condition was reversible if the cultures (either aerobic or anoxic) were not exposed to the accumulated sulfide for more than 2-3 hours. The maximum loading of the biomass, the specific feed rate at which H.sub.2 S breakthrough occurs, was estimated to be 5.4-7.6 mmols H.sub.2 S/h-g biomass under anoxic conditions and 15.1-20.9 mmols H.sub.2 S/h-g biomass under aerobic conditions.
It has also been shown that heterotrophic contamination resulting from septic operation of T. denitrificans cultures has a negligible effect on H.sub.2 S oxidation by the organism. The autotrophic medium used to grow T. denitrificans contained no organic components to support heterotroph growth. Apparently, organic carbon was obtained from waste products of T. denitrificans or cell lysis. It has also been demonstrated that T. denitrificans may be flocculated by aerobic co-culture with floc-forming heterotrophs from an activated sludge system (Ongcharit, C., P. Dauben and K. L. Sublette, "Immobilization of an Autotrophic Bacterium by Coculture with Floc-Forming Heterotrophs," Biotech. Bioeng., 33, 1077-1080 (1989) and Ongcharit, C., K. L. Sublette and Y. T. Shah, "Oxidation of Hydrogen Sulfide by Flocculated Thiobacillus denitrificans in a Continuous Culture," Biotech. Bioeng., 37, 497-504 (1991)). An H.sub.2 S-active, gravity-settleable floc resulted which was used to scrub H.sub.2 S from a gas in a continuous stirred-tank reactor with biomass recycle. T. denitrificans remained flocculated with successive subculturing with no further introduction of floc-forming heterotrophs.
Sour water containing up to 25 mM inorganic sulfide was successfully treated in an aerobic up-flow bubble column (3.5 L) containing 4.0 g/L of flocculated T. denitrificans (Lee, C. and K. L. Sublette, "Microbial Oxidation of Sulfide-Laden Water," Water Research, 27(5), 839-846 (1993)). The sulfide-laden water was supplemented with mineral nutrients only. The sulfide-active floc was shown to be stable for nine months of continuous operation with no external organic carbon required to support the growth of the heterotrophs. The floc exhibited excellent settling properties throughout S the experiment. Retention times in the reactor varied from 1.2-1.8 hrs; however, molar sulfide feed rate (mmols/hr sulfide) was more important in determining the capacity of the reactor for sulfide oxidation than either the hydraulic retention time or the influent sulfide concentration (mM). At a biomass concentration of about 4 g/L, the column could be operated at a molar sulfide feed rate of 12.7-15.4 mmols/h without upset.
Wild-type T. denitrificans is inhibited by sulfide concentrations of 0.1-0.2 mM. However, sulfide-tolerant strains of T. denitrificans have been isolated by enrichment from cultures of the wild-type T. denitrificans. This is done by repeated subculturing at increasing concentrations of sulfide using standard subculturing techniques known in the art. A strain tolerant of sulfide concentrations in excess of 2.5 mM can be obtained (Sublette, K. L. and M. E. Woolsey, "Sulfide and Glutaraldehyde Resistant Strains of Thiobacillus denitrificans," Biotech. Bioeng., 34, 565-569 (1989)).
Other strictly aerobic Thiobacilli have also been shown to oxidize H.sub.2 S to sulfate in a manner similar to T. denitrificans. These species include T. versutus, T. thioparus, T. thiooxidans and T. neopolitanus (Codenhead, P. and K. L. Sublette, Biotech. Bioeng., 35, 1150-1154 (1990)). These strains of Thiobacilli can likely be immobilized by co-culture with the floc-forming heterotrophs under aerobic conditions to produce a similar sulfide-oxidizing, gravity-settleable floc.